Coaxial cables are widely used with audio-visual technology, electronic instrumentation, and radio broadcasting, because they minimize interference with or from signals traveling on the cable. Coaxial cables consist of a wire inner conductor surrounded by a thin cylindrical conducting shield, usually of braided copper (Fig. 21.39). Flexible insulation separates the conductors. A straight length of coaxial cable can be approximated as an infinitely long wire surrounded by a cylindrical shell. Normally the two conductors carry charges of equal magnitude but opposite sign. (Charge actually varies with time and position as signals travel down the cable, but for these problems consider the charge to be fixed and spread uniformly.)

For a coaxial cable in electrostatic equilibrium carrying equal but opposite charges on its two conductors, there?s a nonzero electric field

a. only in the space between the wire and shield.b. in the space between wire and shield, and outside the shield.c. inside the metal conducting wire and shield, as well as between the wires and outside the shield.d. only outside the shield.

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Outer conductor Inner conductor Insulation Insulation FIGURE 21.39 A coaxial cable (Passage Problems 76–79)